doc: add bufio.Reader.Discard to go1.5.txt

Change-Id: I315b338968cb1d9298664d181de44a691b325bb8
Reviewed-on: https://go-review.googlesource.com/2450Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.5.txt

 API additions and behavior changes:
 
+bufio: add Reader.Discard (https://golang.org/cl/2260)
 crypto/cipher: clarify what will happen if len(src) != len(dst) for the Stream interface. (https://golang.org/cl/1754)
 crypto/tls: change default minimum version to TLS 1.0. (https://golang.org/cl/1791)
 encoding/base64: add unpadded encodings (https://golang.org/cl/1511)